This invention relates to monitoring systems for checking electric rotary machines for local overheating, and, more particularly, to a monitoring system for checking a gas cooled electric rotary machine for local overheating of parts such as a stator core covered with organic insulating material which tends to suffer from thermal deterioration.
When the stator core of a gas-cooled generator of a large size develops local overheating, the iron core suffers damage and mechanical failure thereby making it necessary to effect highly expensive repairs. One of the factors concerned in the development of local overheating in this type of electric rotary machine is damage to the surface of the stator which might cause electric contact to be established between layers of the laminated iron core. When this happens while a load is being applied to the machine, a current would flow and heat of resistance would be generated in the iron core. This local overheating would cause a generation of heat of an amount sufficient to melt the laminated iron core. Thus the need to provide the machine with a system for an early detection or sensing of local overheating has been keenly felt, so that a load applied to the machine can be reduced and serious damage can be avoided by taking necessary steps to prevent the overheating from spreading to other parts of the machine.
Generally, an iron core and coil of a generator are covered with organic insulating material, such as phenol resin and, in the event that local overheating develops, the organic insulating material would be exposed to high temperature and undergo pyrolysis that would cause minuscule particles of less than 1.mu. to be produced and dispersed in the coolant or gas. In, for example, Japanese Patent Application Laid-Open Number 64703/75 a device for sensing minuscule particles in a coolant gas is proposed wherein an ionizing section producing corona discharge, and a detecting section forming a weak electric field for collecting ionized particles are provided. With this proposed device it is possible to detect the presence or absence of minuscule particles in the gas by ionizing and causing the ionized particles to be trapped by an electrode so that a current produced by the released charge can be measured. The value of the current obtained in this manner would correspond to the concentration of the minuscule particles in the gas. Thus, by using this detecting device for monitoring the concentration of minuscule particles in the gas serving as coolant at all times, it would be possible to indirectly check the generator for possible local overheating of parts covered with organic insulating material.
When the detecting device of this type is used with an electric rotary machine for monitoring the concentration of minuscule particles and the device malfunctions and generates a wrong signal, the electric rotary machine would be shut down without any valid reason, thereby causing a reduction in the rate of operation of the machine.
To avoid this disadvantage, proposals have been made to use a monitoring system for checking an electric rotary machine for local overheating, which proposals include providing a filter device mounted in a cooling gas extracting passage between the electric rotary machine and the detecting device for filtering the cooling gas to remove minuscule particles therefrom, and a testing device located upstream of the filter device and including a filament covered with organic material.
This monitoring system operates such that the cooling gas extracted from the electric rotary machine is supplied to the detecting device to determine whether or not local overheating is present, and, when the presence of overheating is detected, the flow of cooling gas is switched to the filter device to remove the minuscule particles therefrom. The flow of cooling gas is then introduced into the detecting device and the determination of the presence of overheating is acknowledged as being valid if the value obtained by detecting is found to be at normal background level. When the value determined after filtering the gas fails to return to the normal background level, the condition can be attributed to either a malfunction of the detecting device or malfunction of the filter device.
In order to check the filter device for its performance, a current is passed to the filament of the testing device for a predetermined time to cause the organic material covering the filament to undergo pyrolysis, and the minuscule particles produced by the pyrolysis of the organic material are added to the gas serving as coolant. In this fashion, a test gas similar to the gas containing minuscule particles and causing local overheating in an electric rotary machine is prepared, and tests are conducted by using this test gas. That is, the test gas is directly fed into the detecting device without passing through the filter device, and also supplied to the detecting device by way of the filter device. The values obtained by measuring the minuscule particles in the two gases are compared with each other to check the filter device for its performance.
In this type of local overheating monitoring system, solid minuscule particles are produced by pyrolysis of the testing device during operation and fed together with the coolant gas after clearing the detecting device. Thus, the concentration of the minuscule particles in the machine gradually rises as the testing device is actuated. During normal operation of the electric rotary machine, production of minuscule particles in abnormally large quantities due to pyrolysis stemming from local overheating can be better detected when the concentration of minuscule particles in the machine is low. However, in the event that minuscule particles for which no local overheating is responsible are fed into the machine as aforesaid, then the background level of the concentration of minuscule particles in the coolant gas gradually rises, so that the values obtained by the detecting device are unrealiable.
Also, the testing device has a low reliability, because the contact strength between the organic material and the filament may be reduced when, due to the moisture content of the gas, dew is formed on the surface of the filament and, due to vibration, the organic material may be dislodged.